1. Field of the Invention
The present invention relates to the testing of dialysates, used in kidney dialysis, to confirm that the dialysates are safe for use to cleanse the blood of patients with kidney failure. More particularly, the present invention relates to devices and methods for confirming that the components of dialysates are present in the correct proportions.
2. Description of the Related Art
Dialysates are used in kidney dialysis (hemodialysis) to cleanse the blood of patients with kidney failure. Generally, dialysate is a solution of buffered salts and glucose in purified water. In the majority of dialysates, a bicarbonate ion is the buffering ion. Bicarbonate dialysate is prepared by combining a bicarbonate concentrate with an acid concentrate, and then diluting the mixture with purified water to obtain the correct proportion of the dialysate components. Clinical technicians may prepare the bicarbonate concentrate “on site” at a dialysis facility, but more commonly, the bicarbonate concentrate is purchased along with the acid concentrate from a commercial supplier.
The dialysate is typically prepared by a dialysis machine, which performs the actual combining, mixing and diluting of the bicarbonate and acid concentrates. Dialysis machines generally include a blood pump, a dialysis solution delivery system, and appropriate safety monitors. There are two major types of dialysis solution delivery systems, a central proportioning delivery system and an individual proportioning system. In the central proportioning delivery system, all of the dialysate is produced by a single machine, and the dialysate is then pumped through pipes to individual dialysis machines. In an individual proportioning delivery system, each dialysis machine proportions the dialysate separately. The blood pump moves the patient's blood to a dialyzer where the blood is cleansed with the dialysate. The cleansed blood is returned to the patient, and the used dialysate flows into a drain and is discarded.
If the proportioning system that dilutes the bicarbonate and acid concentrates with water malfunctions, an excessively dilute or concentrated dialysate may be produced. Daugirdas, J. T., Ing, T. S., Handbook of Dialysis, 2nd ed., Little, Brown and Company, Boston/New York/Toronto/London, 1994, p. 48. Exposure of blood to a severely hyperosmolar (too concentrated) dialysate can lead to hypernatremia and other electrolyte disturbances, while exposure to a severely hypoosmolar (too dilute) dialysate can result in rapid hemolysis or hyponatremia. Id. It is therefore critical to ensure that the dialysate is proportioned correctly such that it is safe for use with the blood of a patient before dialysis begins. According to the industry standards, the pH of dialysate should be between 6.0 and 8.0. ANSI/AAMI RD5, § 3.3.1.6 (1992). Additionally, all solutes identified on a concentrate label should be present within +/−5% of the stated concentration or weight, while sodium and chloride, in particular, should be present within +/−2% of the labeled concentration or weight. ANSI/AAMI RD5, § 3.3.1.2 (1992). Generally, the concentration of the ionic components of the dialysate can be indirectly determined by the electrical conductivity of the dialysate, because the primary solutes in dialysates are electrolytes.
Most dialysis machines are equipped with built-in meters or other safety devices that continuously monitor, among other variables, dialysate concentration and pH. The pH of the dialysate is typically measured by means of a glass pH electrode built into the dialysis system. The dialysate concentration is typically determined indirectly by measuring the electrical conductivity of the dialysate with a conductivity meter.
One problem with these safety devices is that both the conductivity meters and the glass pH electrodes require routine maintenance and calibration checks to insure proper operation. Disadvantageously, this maintenance and calibration checks are time consuming, and are often beyond the technical capability of clinic personnel.
Further disadvantages result from the nature of the conductivity and pH measurements. Specifically, because conductivity is a measurement of the total ion concentration in solution, it is therefore a nonspecific measurement of the concentrations of particular ionic components in the dialysate. This nonspecific measurement can fail because both the bicarbonate concentrate and acid concentrate each contain specific ionic components. In some cases, the observed conductivity measurements are correct, when in fact, the proportion of the bicarbonate and acid concentrates is incorrect. For example, if the concentration of one of the concentrates is too high and the other is too low, the concentrate whose concentration is too high will compensate for the concentrate whose concentration is too low. This results in a conductivity measurement that is mistakenly observed as an indication that the dialysate composition is correct. This problem is recognized in the above-referenced ANSI/AAMI standard, which states:
Adequate monitoring does not currently exist to assure that mismatched concentrates will not produce a final dialysate of proper total conductivity but improper composition. The user is cautioned not to rely solely on conductivity measurements to ensure safety, but to consider all relevant factors, including pH.
ANSI/AAMI RD5, § 3.3.1.6 (1992) (emphasis in original). Another concern with the current systems is that pH measurements are also by nature, as logarithmic measurements, insensitive to errors in the proportion of the bicarbonate and acid concentrates in the dialysate. Only substantial changes in the proportion of the bicarbonate and acid concentrates will affect the change of the pH value. For example, at the correct proportion of the bicarbonate and acid concentrates, the calculated pH is 7.6. However, if the amount of acid concentrate is doubled, the pH will drop slightly to 7.3, which is well within the acceptable pH range of 6.0 to 8.0, even though the actual proportion of bicarbonate and acid concentrates is incorrect.
An alternative device for measuring the pH of a solution has been in a form of test strips. Such test strips have been disclosed in the U.S. Pat. No. 3,122,420 issued to Rebar et al. in 1964, and the U.S. Pat. No. 3,232,710, issued to Reickmann et al., in 1966. The '420 patent discloses bibulous paper strips impregnated with a diagnostic composition for use in determining the hydrogen ion concentration of biological fluids such as human urine. The '710 patent, in addition to the pH test paper strips, discloses glucose test paper strips, albumin test paper strips, and multiple test strips. Another glucose test strip appropriate for the detection of glucose in body fluids such as urine has also been disclosed in the U.S. Pat. No. 2,912,309, issued to Free, in 1959.
Despite the different kinds of test strips listed above, there has not been a test strip that can be used in determining the proportion of bicarbonate and acid concentrates in dialysate.
What is needed is a test for determining whether the bicarbonate and acid concentrates in dialysate are present in the correct proportion.
What is also needed is a test that will enable users to obtain a quick, reliable, and visual qualitative determination of whether bicarbonate and acid concentrates are correctly proportioned in dialysate.
A further need is for tests that may be performed by clinical personnel who do not possess advanced scientific and/or technical training.